Although electronic signal processing techniques are effective at frequencies below 1 to 2 GHz, they are of limited applicability at higher frequencies. Single-mode optical fiber, on the other hand, is an excellent frequency-independent delay medium (0.2 km/.mu.sec), with demonstrated modulation bandwidth &gt;100 GHz.km and low loss (&lt;0.2 dB/km). The low loss, large bandwidth, and the small size associated with single mode optical fiber make it an attractive choice as a delay line to implement signal processing functions at microwave frequencies. As a result, use of such a fiber can form the basis of signal processing elements offering orders of magnitude increase in bandwidth over electrical devices.
Signal processing optical fiber devices are schematically, structurally and operationally similar to their electronic counterparts. The design, architectures and analysis are also essentially equivalent. However, the fundamental bending loss limitation inherent in recirculating delay line structures has impeded the use of optical fiber for such delay applications as performing a bandpass filter function.
Many processing operations using basic tapped and recirculating delay lines together with more complex feed-forward and feed-backward lattices, have already been demonstrated. K. P. Jackson, et al., "Optical fiber delay-line signal processing," IEEE Trans. Microwave Theory Tech., Vol. MTT-33, p. 193, (1985). Simple tapped delay lines carry out basic transversal filter operations (convolution, correlation, matched filter and code generation) as well as bandpass filters and notch filter operations at frequencies above 1 GHz. See K. P. Jackson, et al., "Microbend optical fiber tapped delay line for gigahertz signal processing," Appl. Phys. Lett., Vol. 41, p. 139, (1982); J. E. Bowers, et al., "Filter response of single-mode fiber recirculating delay lines," Electron. Lett., Vol. 18, p. 110, (1982); S. A. Newton, et al., "Optical fiber V-groove transversal filter," Appl. Phys. Lett., Vol. 43(2), p. 149, (1983); C. C. Wang, "Cascaded single-mode fiber optic transversal filters," Proc. SPIE High Frequency optical communication, Vol. 716, p. 82, (1986). Recirculating delay lines are capable of temporary data storage and data rate transformation and have been demonstrated as frequency filters above 1 GHz. Fiber-lattice structures can perform matrix-vector multiplication at 100 MHz and broadband filtering at frequencies in excess of 1 GHz. B. Moslehi, et al., "Fiber-optic lattice signal processing," Proc. IEEE, Vol. 72, p. 909, (1984).
Extension to frequencies of 10 GHz, and above, is straightforward in principle. However, it requires the use of shorter fiber lengths, more compact designs and faster optoelectronic interfaces. The designs to date result in processors which operate at much lower frequencies. In addition, in a straightforward implementation, bending losses in the fiber place a lower limit on the length of the loop. For example, in a tapped fiber delay line of the type disclosed in U.S. Pat. No. 4,558,920 to Newton, et al., the tapping points are usually separated by a distance equal to the circumference of the cylinder around which the optical fiber is looped. The bending loss will limit the minimum loop length (cylinder circumference) and thus limit the high frequency of operation. For higher frequencies, it is necessary to consider alternative designs and architectures, e.g. multiple tap points per fiber loop around the cylinder, linear tap configurations, or a recirculating lattice. The present invention considers a recirculating lattice approach.